Insulating coating



Aug. 29, 1939. D. H. WAMSLEY 2,171,230

INSULATING COATING Filed Feb. 28, 1958 Z 6 1-H 4 q 15 w I. I

ATTORNEY.

Patented Aug. 29, 1939 PATENT OFFICE INSULATING COATING Delos H.Wamsley, West Caldwell, N. J., assignor, by mesne assignments, to RadioCorporation of America, New York, N. Y., a corporation of DelawareApplication February 28, 1938, Serial No. 193,137

4 Claims.

My invention relates to electron discharge devices, and to means forsubstantially eliminating secondary electron emission from the surfacesin such tubes exposed to primary electron bombardment, and particularlyto the prevention of secondary electron emission from and surfaceleakage along insulators and insulating spacers for electrodes in suchdevices.

It is common practice to use mica or ceramic spacers in electrode mountsto insulatingly space the electrodes. It has been proposed to coat theinsulating surfaces of envelopes and spacers of radio tubes withpowdered refractory metal compounds to increase the elfective electricaldistance between points on the surface and thereby reduce leakage. Thecompounds heretofore used for such coating are ineffective in preventingsecondary electron emission from the coated surface.

An object of my invention is to provide coating material to reducedirect leakage over the surfaces of insulators, such as spacers, andwhich also prevents electron emission from the coated surfaces.

A better understanding of my invention may be obtained by referring tothe following specification in which preferred embodiments are describedfor purposes of illustration.

Figure lrepresents a conventional tube with insulating surfaces coatedwith a layer of an alumina hydrate according to my invention and Figure2 shows an enlarged view of an insulator for an electron dischargedevice coated in ac cordance with my invention with an alumina hydrate.

For purposes of illustrating my invention an electron discharge devicewith a conventional glass envelope l is shown enclosing any desiredelectrode arrangement including a cathode 2. Insulating surfaces of theenvelope and of mica spacers 3 and 4 are coated with a material 5 forreducing secondary emission. The coating as shown in Figure 2 may beapplied in an adherent uniform layer on any of the insulator surfaceswhich may receive electrons from the cathode.

Insulating surfaces of an electron discharge device including thesurfaces between metal conductors of different potentials, such as theenvelope wall adjacent the lead-in seals, grids, and electrode spacersof the mica or ceramic type, as well as grids and metal parts arecoated, according to my invention, with a layer of alumina monohydrate,A12O3-H2O. The alumina monohydrate is preferably powdered to a particlesize of less than 50 microns, and suspended in water or in a volatileliquid, such as nitrocellulose. The suspension may be applied to thesurfaces by spraying, dipping or painting and the surfaces dried toleave on them a. rough adherent coating of alumina monohydrate. It hasbeen found that a coating approximately .005" thick on the surfaces willsuppress secondary electron emission and will also prevent leakage overinsulators or insulating surfaces, such as the mica spacer in theconventional type of tube shown for example in United States Patent1,672,233 to Skinner. The coated parts are assembled in the electrondischarge device and during exhaust the parts are heated in the usualmanner to drive out occluded gases.

An electron discharge device constructed in accordance with my inventionhas practically no secondary electron emission from the coated surfaces,and has very low leakage currents over its insulating surfaces betweenconductors of different potentials. The operation of my tube in highfrequency currents is particularly stable because of the absence ofemission of secondary electrons from surfaces coated with my novelinsulating coating material.

I have found that alumina monohydrate coated on insulating surfaces notonly practically eliminates secondary electron emission from thesurfaces, but also substantially reduces leakage currents over thesurfaces, particularly in tubes having cathodes coated with barium andstrontium oxides. In such tubes it seems probable that active metals,such as barium, often evaporate from getters or the oxide coated cathodeand condense on insulating surfaces in the usual electron dischargedevice, producing troublesome leakage paths and sources of secondaryelectrons on the surfaces. It is my belief that alumina monohydratecoated on insulating surfaces in accordance with my invention reactswith the active metals condensed thereon to form inert compounds whichare resistive to leakage currents and prevent emission of secondaryelectrons. Tests indicate, and it is my belief, that alumina monohydratecoated on the insulating surfaces in accordance with my inventionretains its water of crystallization during manu facture and exhaust ofthe electron discharge device. Barium depositing on the aluminamonohydrate is probably converted into barium hydroxide, which is aneffective insulator for leakage currents and is particularly inert as asource of secondary electrons. As alumina monohydrate is a crystallinematerial, which when sprayed and dried forms a roughened granularcoating, the reduction in leakage is apparently due to an increase inlength of leakage paths over the surface.

I claim:

1. An electron discharge device having cooperating electrodes in anenvelope, insulating surfaces in said envelope, and a coating of aluminamonohydrate at least .005" thick on said surfaces.

2. An electron discharge device, an insulator in said device, and acoating of alumina monohydrate 0n the surface of said insulator.

3. An electron discharge device comprising an envelope and cooperatingelectrodes in said envelope, a coating of an alumina hydrate oninsulating surfaces in said envelope to reduce leakage currents over andsecondary emission from said surfaces.

4. An electron discharge device with an envelope containing an oxidecoated cathode and cooperating electrodes, comprising means forpreventing emission of secondary electrons from some of the surfaces inthe envelope of said electron discharge device comprising a coating ofalumina monohydrate on said surfaces.

DELOS H. WAMSLEY.

